1. Field of the Invention
The invention relates generally to oscillator circuits and more specifically to a high power, high efficiency oscillator circuit.
2. Related Art
Class F amplifiers are well known in the art and are described in various articles and books including xe2x80x9cFET Power Amplifier with Maximally Flat Waveform,xe2x80x9d, by F. H. Raab, IEEE Trans., MTT, Vol. 45, No. 11, Nov. 1997, pp. 2007-2011, and
RF Power Amplifiers for Wireless Communications, by S. C. Cripps, S. C., Norwood, M A: Artech, 1999, ISBN 0-89006-989-1. The state of the art for such amplifiers is believed to be about 50 watts and 65% efficient at somewhat less than 300 MHz frequency of operation.
The inventors are unaware of any reference which describes a power oscillator which utilizes class F amplifier principles. Class F amplifiers typically operate at a frequency range of less than 300 MHz and use lumped multiple-resonator output filters to control the harmonic content of their drain-voltage and/or drain-current waveforms. With an unstable load, these conventional Class F tank circuits develop high voltages which would be potentially destructive if fed back to the input (e.g. gate) of a solid state active device. Accordingly, the well known Class F drain circuits are not well suited for an oscillator configuration, particularly at high power and dynamically varying load conditions, such as those presented by an electrodeless lamp.
The following and other objects, aspects, advantages, and/or features of the invention described herein are achieved individually and in combination. The invention should not be construed as requiring two or more of such features unless expressly recited in a particular claim.
In general, the present invention relates to the type of power oscillators described in PCT Publication Nos. WO 99/36940 and WO 01/03161, each of which is herein incorporated by reference in its entirety. The power oscillators described in these references are believed to define the state of the art in terms of power output, frequency range, and efficiency.
According to one aspect of the invention, an oscillator includes a solid state active device having an input and an output; and a feedback circuit connected from the output of the active device to the input of the active device, the feedback circuit providing suitable positive feedback to initiate and sustain an oscillating condition at a fundamental frequency; characterized in that the oscillator further includes a waveform modifying circuit connected to the output of the active device, wherein the waveform modifying circuit is adapted to modify the waveform on the output in a manner which increases an efficiency of the oscillator. For example, the waveform modifying circuit comprises transmission lines and optionally components which provide a high impedance for odd harmonics of the fundamental frequency. The waveform modifying circuit may further include transmission lines and optionally components which provide a low impedance for even harmonics of the fundamental frequency.
According to another aspect of the invention, an oscillator includes a solid state active device having an input and an output; and a feedback circuit connected from the output of the active device to the input of the active device, the feedback circuit providing suitable positive feedback to initiate and sustain an oscillating condition at a fundamental frequency; characterized in that the oscillator further includes a transmission line coupled to the output of the active device, the transmission line having an effective electrical length of one twelfth of a wavelength of the fundamental frequency. The oscillator may further include another transmission line connected to the output of the active device and having an effective electrical length of one eighth of the wavelength of the fundamental frequency.
According to another aspect of the invention, a high power solid state power RF oscillator includes a power FET having a gate, a drain, and a source, wherein the source is RF grounded and suitable voltages are respectively applied to the drain and gate to place the transistor in a conducting condition; a drain circuit connected to the drain of the power FET; and a feedback circuit connected between the drain circuit and the gate, wherein the feedback circuit is adapted to provide suitable positive feedback to initiate and sustain an oscillating condition with a fundamental frequency being greater than 300 MHz and with an RF power output in excess of 50 watts; characterized in that the drain circuit comprises transmission lines having widths which provide a conjugate match for the drain at a fundamental frequency and lengths which provide a high impedance for odd harmonics of the fundamental frequency. The drain circuit may further include a transmission line having a length which provides a low impedance for even harmonics of the fundamental frequency. For example, the drain circuit comprises a first transmission line connected at a first end to the drain and at a second end to an output matching circuit, and a second transmission line which is an open stub connected to the second end of the first transmission line, wherein the first transmission line and second transmission lines each have a width which provides a conjugate match for the drain at a fundamental frequency and an effective electrical length of about one twelfth of a wavelength of the fundamental frequency. The drain circuit may further include a third transmission line connected at one end to the drain and having an effective electrical length of one eighth of the wavelength of the fundamental frequency.